重新指定字段字符集即可(排序规则默认)

CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `serial_no` varchar(50) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

alter table t1 modify `serial_no` varchar(50) CHARACTER SET utf8mb4 NOT NULL;

show create table t1;
/*
CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `serial_no` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
*/

mysql 有语句可以重新指定全表字符集。

CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `serial_no` varchar(50) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

执行语句：

ALTER TABLE t1 CONVERT TO CHARACTER SET utf8mb4;

CREATE TABLE t1 (
id int NOT NULL AUTO_INCREMENT,
serial_no varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

官方文档介绍
To change the table default character set and all character columns (CHAR, VARCHAR, TEXT) to a new character set, use a statement like this:

ALTER TABLE tbl_name CONVERT TO CHARACTER SET charset_name;